FEXCore custom CPUID functions

4000_0000h - Hypervisor information function

• Follows VMWare and Microsoft's hypervisor information proposal

• https://lwn.net/Articles/301888/

• https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/feature-discovery

• EAX - The maximum input value for the hypervisor CPUID information

  • 4000_0001h

• EBX - Hypervisor vendor ID signature

  • 'FEXI' - 4958_4546h

• ECX - Hypervisor vendor ID signature

  • 'FEXI' - 4958_4546h

• EDX - Hypervisor vendor ID signature

  • 'EMU\0' - 0055_4d45h

• memcpy ebx:ecx:edx in to a 12 byte string to get 'FEXIFEXIEMU\0' for determining running under FEX

4000_0001h - Hypervisor config function

Sub-Leaf 0: ECX == 0

• EAX:
  • Bits EAX[3:0] - Host architecture
    • 0 - Unknown architecture
    • 1 - x86_64
    • 2 - AArch64
    • 3-15: Reserved
  • Bits EAX[15:4] - Reserved
  • Bits EAX[31:16] - Maximum subleaf input value for CPUID function 4000_0001h
• EBX - Reserved - Read as zero
• ECX - Reserved - Read as zero
• EDX - Reserved - Read as zero

Sub-leaf 1: ECX == 1

• FEX version string signature. First 16-bytes
• memcpy eax:ebx:ecx:edx in to the first 16-bytes of a string.

Sub-leaf 2: ECX == 2

• FEX version string signature. Second 16-bytes
• memcpy eax:ebx:ecx:edx in to the second 16-bytes of a string.

Sub-Leaf 0000_0003 - FFFF_FFFF: Reserved

4000_0002h - 4000_000Fh

• Reserved range
• Returns zero until implemented

4000_0010h - 4FFF_FFFFh

• Undefined
• FEX-Emu will return zero until implemented